Acquiring nukes rebalances the power between Iran and the US and its neighbors. Roughly equal pairs of states fight more than lopsided ones, historically. The probability for a win on either side is higher when on equal footing.
Nukes as a deterrent caps escalation. This emboldens the holder of the nukes in lower levels of conflict. See Russia with Ukraine (knowing NATO wouldn’t respond), Kargil after Pakistan got nukes, and NK sinking the Cheonan.
Iran getting a bomb probably triggers Saudi Arabia to follow suite, possibly Turkey and Egypt too. Many of the conflicts in the area aren’t (solely) defined by US presence. The Iran–Saudi, Sunni–Shia, and Israel–Iran rivalries to name a few.
It’s possible the dynamics shake out differently, but color me skeptical.